FIG. 1 shows a block circuit diagram for PWM control of a conventional synchronous motor using permanent magnets. FIG. 2 shows an outline of functions of the transistor PWM control circuit in the apparatus of FIG. 1. In FIG. 1, reference numeral 1 denotes a three-phase power source, 2 a rectification circuit, 4 a transistor inverter, 5 a transistor PWM control circuit, 6 a synchronous motor using permanent magnets as a rotor, and 7 a rotor rotation detector, for example, a pulse encoder, for detecting a position and speed of the rotor of the synchronous motor 6. The transistor PWM control circuit 5 compares a velocity command value V.sub.0 with a present velocity V (speed) detected by the rotor rotation detector 7, switches 0N or OFF respective transistors 411 to 416 in the transistor inverter 4, and controls the speed by controlling the electric current through windings having U, V, and W phases in the synchronous motor 6.
As shown in FIG. 2, the transistor PWM control circuit mainly comprises a velocity command value to a present velocity value comparator, an armature current detector, a multiplier for generation of an armature current command value, a phase shifter, a differential amplitude filter for generation of an armature current command value, a feedback portion for saturation detection, and an inverter circuit control signal generator.
The operation of the synchronous motor in the apparatus of FIG. 1 is explained by using the vector diagrams shown in FIG. 3 and FIG. 4A through FIG. 4D. In FIG. 3, the radius V of the circle shows the peak value of the supply voltage to the synchronous motor, and the reference symbol I.sub.a shows an effective current component in the armature winding, K.sub.1 shows a countervoltage constant, K.sub.2 shows a constant corresponding to the pole number, .omega. shows an angular velocity of the rotation of the motor, and L shows an inductance of the armature winding for each phase, respectively. If the total magnetic flux is .PHI., the number of turns of the stator winding is N, the number of poles is P, and a predetermined constant is K.sub.3, and the equation K.sub.1 =.PHI..multidot.N.multidot.P.multidot.K.sub.3 (V.multidot.sec/rad) is satisfied. Also, K.sub.2 =1/2.multidot.P. Since, when the sum of the K.sub.1 .omega., i.e., velocity voltage, and the K.sub.2 .multidot..omega.L.multidot.I.sub.a, i.e., the voltage perpendicular to the K.sub.1 .multidot..omega., which is caused by the electric current I.sub.a and the inductance L, exceeds the circle V, the motor cannot be driven, thus, a reactive current component I.sub.b is added and the voltage K.sub.2 .multidot..omega..multidot.L.multidot.I.sub.b is added. As a result, by leading the driving voltage into circle V, the output power of the motor can be increased. The application of the voltage K.sub.2 .multidot..omega..multidot.L.multidot.I.sub.b as above-mentioned is carried out by the above-mentioned saturation detection circuit 57, ROM 514, 515, D/A converter 521, 522, phase shift circuit 541, 542, 543.
In the above-mentioned conventional apparatus, however, a problem arises wherein when the sum of each voltage vector reaches a point on a semicircle at a side opposite to the center line (shown by a dot-dash line perpendicular to the vector K.sub.1 .multidot..omega.) as shown in FIG. 4B, FIG. 4C, and FIG. 4D, the voltage component caused by the effective current is decreased, and the output power of the synchronous motor is reduced.
The conventional synchronous motor control method using a pulse width modulation signal was described, for example, in U.S. patent application Ser. No. 811,840 filed Dec. 12, 1985. (corresponding to International Patent Application PCT/JP85/00207) by the applicants of the present application.